It's next gen Halo multiplayer... do I need to say anything else? It's basically the only reason most people will be buying the Master Chief Collection.
https://www.youtube.com/watch?v=qqrC0-_QG9A

"Halo 5: Guardians" Multiplayer Beta -- Be among the first to experience a new generation of "Halo" multiplayer in the "Halo 5: Guardians" Multiplayer Beta. Gear up for the "Halo 5: Guardians" Multiplayer Beta by unlocking exclusive content in Halo: The Master Chief Collection and Halo: Nightfall. These exclusive items will carry over into Halo 5: Guardians.